Late years, in connection with the advent of the multimedia information society with the popularization of portable telephones, motion picture communications, electronic commerce and others, the transmission volume of communication networks is being continuously increased. As a technique capable of sending and receiving a large amount of information communication at a lower cost, there has been employed a transmission system based on a wavelength division multiplexing transmission technology comprising giving different informations respectively, to lights different in wavelength, multiplexing the lights with the different wavelengths, and transmitting the wavelength multiplexed light through a single optical fiber. This transmission system uses a filter element for multiplexing/demultiplexing light, an optical filter or switch for selectively picking up a light having a specific wavelength from a wavelength multiplexed light including a plurality of wavelengths, and other components.
As one of the filter elements for use in the above system, there has been known an optical demultiplexer as shown in FIG. 1. In this optical demultiplexer, a plurality of interference film filters 101, 102, 104 adapted, respectively, to reflect lights of different wavelengths are disposed on the optical path of a wavelength multiplexed light 106 transmitted through an optical fiber, so as to reflect lights of specific wavelengths λ1, λ2, λ3 included in the wavelength multiplexed light 106, respectively, to obtain monochromatic lights 108, 110, 112 of specific wavelengths λ1, λ2, λ3.
There has also been known an optical demultiplexer as shown in FIG. 2, which comprises in combination a star coupler 201 and a plurality of interference film filters 202, 204, 206, 208. In this optical demultiplexer, the star coupler 201 for branching light is disposed on the optical path of a wavelength multiplexed light 210 transmitted through an optical fiber, and the interference film filters 202, 204, 206, 208 adapted, respectively, to allow only lights of wavelengths λ1, λ2, λ3, λ4 to be transmitted therethrough are disposed on corresponding branched optical paths.
When the above optical demultiplexers are used in such a manner that the direction of the optical path thereof is reversed, they can act as an optical multiplexer for multiplexing monochromatic light flux.
The above optical demultiplexer, optical multiplexer, or multi/demultiplexer essentially includes filters of the same number as that of multiplexed wavelengths. Thus, they involve a high-cost problem due to increase in the number of components and the number of assembling steps, and the need for complicated adjustments. Further, the optical demultiplexer using a star coupler involves a problem of degradation in output light intensity due to branching of a multiplexed light through the star coupler.
As optical filters or optical switches, there have been developed an optical filter having a grating disposed in a Mach-Zehnder interferometer type optical waveguide, and a thermooptic optical switch provided with a thin-film heater which is composed of a thin layer of metal, such as a chromium or copper, vapor-deposited on the above optical filter, and designed to generate heat in response to current supplied thereto to heat the optical waveguide.
However, the optical filter and the optical switch having the above-described optical waveguide is not suited for mass production due to its complicated production process. Further, since it is required in the process of forming the grating to periodically form notches in a core or clad with a high precision in the order of several microns or more, the cost is increased.
In view of the above circumstances, it is therefore an object of the present invention to provide an optical multi/demultiplexer having a reduced number of components and allowing the number of assembling steps to be reduced while eliminating the need for complicated adjustments.
It is another object of the present invention to provide an optical filter capable of being readily produced and varying the wavelength of a monochromatic light to be picked up.